This invention relates to preparing bonded, corrosion resistant coated, metal substrates which are resistant to delamination and free of hexavalent chromium in the corrosion resistant coating.
The structural bonding of metal to metal and composite type assemblies widely used in the aircraft industry and elsewhere frequently require a resultant structure which is reasonably resistant to the extremes of atmospheric conditions found in use. To avoid failures of the aircraft structures, bonded metal to metal and composite type assemblies must be able to withstand the environmental conditions to be encountered. Of particular importance is resistance to corrosion and delamination of composite structures. Heretofore, the adhesively bonded metal-to-metal and composite type assemblies (absent a chromated primer) have performed less than satisfactorily due to adhesive failure at the interface between the polymeric adhesive and the aluminum surface.
Conversion coatings have been widely used in metal surface treatment for improved corrosion inhibition. Conversion coatings are applied through chemical reactions between the metal and the bath solution which converts or modifies the metal surface into a thin film with required functional properties. Conversion coatings are particularly useful in surface treatment of metals such as steel, zinc, aluminum and magnesium. In the past, chromate conversion coatings have proven to be the most successful conversion coatings for aluminum and magnesium. However, chromate conversion coatings used in the past generally contained highly toxic hexavalent chromium. The use of hexavalent chromium results in potential hazardous working conditions for process operators and very high costs for waste disposal.
It is highly desirable to provide an improved process for preparing corrosion resistant, environmentally friendly, bonded metal substrates which are resistant to delamination.